Polyethylene films have established a major position in the food packaging industry due to their excellent shelf life, product protection, product display, and low cost. The characteristic of the packaged food product determines the optimal barrier performance of the packaging materials. Optimal barrier for some food products requires high barrier materials, while others need low barrier materials. For instance, dry foods, such as cereals, crackers, cookies and powdered products, require the packaging materials have high barrier to water vapor or moisture, while poultry products require the packaging materials have high barrier to oxygen.
High density polyethylene (HDPE)-based films are the material of choice for many of these packaging applications. HDPE layers provide desirable physical properties such as impact resistance, tear strength and tensile properties to the multilayer films. However, HDPE films do not have sufficient barrier properties for food packaging. Thus, the multilayer films often include one or more barrier layers. Commonly used barrier resins include ethylene-vinyl acetate (EVA) copolymers, ethylene-vinyl alcohol (EVOH) copolymers, nylons and the like. Barrier resins usually cost more than HDPE. Therefore, there is a need to improve the barrier properties of HDPE films. HDPE films of improved barrier properties may replace more expensive multilayer barrier films. Alternatively, HDPE films of improved barrier properties may be used as layers in multilayer barrier films to reduce the use of more expensive barrier resin layers.
Nucleating agents are commonly used in polypropylene. It changes the crystallization temperature, spherulitic size, density, clarity, impact and tensile properties of polypropylene. Similarly, nucleating agents are also used in polyethylene, particularly in linear low density polyethylene (LLDPE) to improve optical, impact, and other physical properties. However, the use of nucleating agent in high density polyethylene (HDPE) is less common because HDPE readily crystallizes without nucleating agent. In general, nucleating agents do not significantly improve the barrier properties of HDPE films.